Memory is built out of tiny capacitors. They get charged if a bit is "1" and discharged if a bit is "0". Because they are capacitors, they have to be refreshed. I don't know what the timings are like precisely on modern memory, but back in the days of 30-pin SIMMs, the timing was 60ns access (16 MHz) and 60 microseconds refresh time, for decent quality memory.
This means that the capacitors needed to be refreshed at least every 60 microseconds, in order to retain their data. Otherwise, they would discharge and all the bits would go to "0".